1. Field of the Invention
The present invention generally relates to an ultrasonic imaging system for transmitting ultrasonic pulses with a transmission frequency in accordance with a pulse repetition frequency (PRF) to a biological body under medical examination, and for receiving ultrasonic echoes reflected from the scanned interior of the biological body to obtain an ultrasonic image of this interior. More specifically, the present invention is directed to such an ultrasonic imaging system capable of varying a maximum pulse repetition frequency based upon various conditions such as ultrasonic pulse transmission frequencies and field depths.
2. Description of the Prior Art
In general, when ultrasonic pulses are transmitted from an ultrasonic probe of a conventional ultrasonic imaging system to a biological body under medical examination, and ultrasonic echoes reflected from the scanned interior of the biological body are received by this ultrasonic probe to image a sectional view of tissues within this interior, if a pulse repetition frequency (PRF) is preset to a high frequency with respect to the transmission frequency "f.sub.L " of the ultrasonic pulses, residual echoes happen to occur in the received echoes. Then, the residual echoes may cause qualities of the ultrasonic images to be deteriorated. An occurrence of such a residual echo will now be described more in detail. That is, as represented in FIG. 1, during a first PRF term, first ultrasonic (UL) pulses are transmitted and also first echoes are received. However, since the first echoes are still present even when a second PRF term is commenced (namely, second ultrasonic pulses are being transmitted), the rear portions of these first echoes are mixed with the front portions of the second echoes, which produces so-called "residual echoes". Accordingly, in order not to produce or receive such residual echoes, the upper limit value of this pulse repetition frequency (PRF) is preset. This upper limit value is fixed for each of the conventional ultrasonic imaging systems. Accordingly, the ultrasonic pulses and the ultrasonic echoes are transmitted/received in accordance with a PRF value lower than this fixed upper limit value in the conventional ultrasonic imaging system.
However, the above-described conventional ultrasonic imaging system has the following drawbacks. That is, generally speaking, an occurrence of such residual echoes depends upon a transmission frequency (f.sub.L) of ultrasonic pulses, and several sorts of ultrasonic probes having different transmission frequencies such as 3.5 MHz, 5 MHz and 7 MHz are utilized within a single ultrasonic imaging system. However, as previously explained, since the upper limit value of the pulse repetition frequency has been preselected to one fixed value (constant PRF value) even when these different transmission frequencies are utilized in a single ultrasonic imaging system, the reception time period within the PRF term is not always effectively utilized. This second problem will now be explained more in detail with reference to FIG. 2. In FIG. 2, it is assumed that a first ultrasonic probe with a first transmission frequency "f.sub.1 " is employed at a first examination stage under the fixed PRF value. In this case, for instance, since the first echoes can be completely received within the first PRF term, neither the above-mentioned residual echoes, nor idle time "IT" is produced. Thereafter, a second ultrasonic probe with a second transmission frequency "f.sub.2 " is employed to replace the first ultrasonic probe with the first transmission frequency "f.sub.1 ". It should be noted that the second transmission frequency "f.sub.2 " is higher than that first transmission frequency "f.sub.1 ". As apparent from FIG. 2, the first echoes produced under the second transmission frequency "f.sub.2 " are attenuated within a short time period, as compared with the first echoes produced under the first transmission frequency "f.sub.1 ". As a consequence, an idle time "IT" is produced before the subsequent second ultrasonic pulses are transmitted from the second ultrasonic probe with the second transmission frequency "f.sub.2 ".